Check Checksum:

-----------------------------------------------------------------------------
Eiffel
-----------------------------------------------------------------------------

from
	i := 1
	j := 0
until
	i > string.count
loop
	a_char := string.item (i)
	if a_char.is_alpha or a_char.is_digit then
		checksum := checksum.bit_xor (a_char.code.bit_shift_left ((j \\ 4) * 8))
		j := j + 1
	end		
	i := i + 1
end


-----------------------------------------------------------------------------
PHP
-----------------------------------------------------------------------------

foreach ($_REQUEST as $key => $value)
{
	if($key != checksum)
		$data_for_checksum .= $value
}

$data_for_checksum = eregi_replace("|","", $data_for_checksum)


$len = strlen($data_for_checksum);
for($i=0, $j=0, $checksum=0; $i<$len; $i++){
	$p = ord($data_for_checksum[$i]);
	// if char is alpha or digit
	if (($p >= 48  && $p <= 57) || ($p >= 65 && $p <= 90) || ($p >= 97 && $p <= 122))
	{
		$checksum ^= (ord($data_for_checksum) << ((j%4)*8));
		$j++;
	}
}
// todo: check if $checksum is the same as the one sent